2.8: How to add a button to the toolbar?
Campbell added some stuff to the API and had I been using the latest build I would have had access to a new code template:

# This example adds an object mode tool to the toolbar.
# This is just the circle-select and lasso tools tool.
import bpy
from bpy.utils.toolsystem import ToolDef
from bpy.types import WorkSpaceTool

class MyTool(WorkSpaceTool):
    bl_space_type='VIEW_3D'
    bl_context_mode='OBJECT'

    # The prefix of the idname should be your add-on name.
    bl_idname = "my_template.my_circle_select"
    bl_label = "My Circle Select"
    bl_description = (
        "This is a tooltip\n"
        "with multiple lines"
    )
    bl_icon = "ops.generic.select_circle"
    bl_widget = None
    bl_keymap = (
        ("view3d.select_circle", {"type": 'LEFTMOUSE', "value": 'PRESS'},
         {"properties": [("wait_for_input", False)]}),
        ("view3d.select_circle", {"type": 'LEFTMOUSE', "value": 'PRESS', "ctrl": True},
         {"properties": [("mode", 'SUB'), ("wait_for_input", False)]}),
    )

    def draw_settings(context, layout, tool):
        props = tool.operator_properties("view3d.select_circle")
        layout.prop(props, "mode")
        layout.prop(props, "radius")


class MyOtherTool(WorkSpaceTool):
    bl_space_type='VIEW_3D'
    bl_context_mode='OBJECT'

    bl_idname = "my_template.my_other_select"
    bl_label = "My Lasso Tool Select"
    bl_description = (
        "This is a tooltip\n"
        "with multiple lines"
    )
    bl_icon = "ops.generic.select_lasso"
    bl_widget = None
    bl_keymap = (
        ("view3d.select_lasso", {"type": 'LEFTMOUSE', "value": 'PRESS'}, None),
        ("view3d.select_lasso", {"type": 'LEFTMOUSE', "value": 'PRESS', "ctrl": True},
         {"properties": [("mode", 'SUB')]}),
    )

    def draw_settings(context, layout, tool):
        props = tool.operator_properties("view3d.select_lasso")
        layout.prop(props, "mode")


def register():
    bpy.utils.register_tool(MyTool, after={"builtin.scale_cage"}, separator=True, group=True)
    bpy.utils.register_tool(MyOtherTool, after={MyTool.bl_idname})

def unregister():
    bpy.utils.unregister_tool(MyTool)
    bpy.utils.unregister_tool(MyOtherTool)

if __name__ == "__main__":
    register()